Field of the Invention
The invention relates to a device for controlling a motor vehicle occupant protection system. The device includes a control unit having an encoder outputting encoded messages and a transmitter for transmitting the encoded messages as output signals.
Such a device is disposed separately, i.e. for example in a side part of the motor vehicle and is described in European Patent Application EP 0 693 401 A2. The known device includes an acceleration sensor and a control unit with an encoder for encoding messages and a transmitter for transmitting the encoded messages in the form of signals. The messages to be transmitted are transmitted in pulse-width-modulated form to an evaluation device disposed centrally within the vehicle. Such a message may for example have the content: "The occupant protection system is to be triggered, because the recorded acceleration has exceeded a predetermined threshold value". Occupant protection system ignition elements connected to the evaluation device, such as, for example, lateral airbags, belt tighteners, head airbags or the like are acted upon with energy, due to an encoded message being transmitted without error, so that the associated occupant protection systems are activated. In order to recognize a defective transmission between a separate device and the central evaluation device, the duration of the period of the transmitted signals is compared with a theoretical period duration in the evaluation device. If the measured period duration and theoretical period duration are not concordant, the transmitted message is discarded. A defective transmission can be caused, in particular by electromagnetic fields acting on the data line.
Since a signal representing that a message has been sent by the separate device is periodically sent to the central evaluation device, a defective signal is at least prevented from being permanently ignored. However, if the signals transmitted subsequently to a defective signal are also defective then, specifically in the case of signals that have triggering instructions as their content, valuable time is wasted for triggering the occupant protection system. In particular, the triggering of an occupant protection system for side-impact protection can subsequently take place too late and cause injuries to the occupant. Because the time required for the triggering of occupant protection systems for side-impact protection are set extremely short, within 5 msec from the commencement of side impact, the triggering decision should be made, since there is no crumple zone protecting the occupant. The triggering time cannot be obtained in time when several defective signals are transmitted one after the other.